A medical ultrasound imaging system is disclosed in U.S. Pat. No. 5,993,390 which makes it possible to acquire a three dimensional image of an organ. In the example disclosed, the organ is the heart. Such a system cooperates with a matrix of more than a thousand of piezoelectric elements with associated coaxial cables, which are called transducers forming a transducer's array, the transducers allowing to send ultrasound scan lines through the organ. It means that a piezoelectric element allows to transmit an ultrasound pulse, to receive a pressure wave and to convert it into an electrical signal. A scan line is composed of a plurality of electrical signals coming from the piezo-electric elements. This transducer's array and its electronics form an ultrasonic probe which is applied onto a patient body near the organ to be imaged. Hence, an ultrasound image is based upon all the ultrasound scan lines and interpolations between said scan lines, both allowing a definition of said image in different gray levels. The 3D images are usually used to detect a defect on the organ.
One drawback of said imaging system is that, in order to check an entire organ such as the left ventricle of the heart, with a good image quality, the 3D image rate is low because of the laws of physics governing the acquisition, in particular the speed of sound of 1540 m/sec. Typically, the image rate is around 20 Hz (that means only twenty images per second). Indeed, the image acquisition rate is function of:                The field of view,        The number of scan lines,        The scan lines density, and        The depth of the image.        
In order to decrease the time of acquisition, and thus to increase the image rate, one can decrease the number of scan lines or decrease the scan lines density. But unfortunately, this solution leads to a very low resolution of the image. It is therefore difficult to detect any defect on the organ via the use of the 3D ultrasound image at high image rate.